The present invention relates to a toner for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like. More particularly it relates to a toner whose electric conductivity sharply changes, depending upon the intensity of the electric field applied thereto, at a predetermined breakdown electric field, thus development of latent electrostatic images and image transfer of the developed toner images can be effectively performed under application of electric fields having different intensities.
As a conventional method of developing latent electrostatic images, there is known a developing method using a one-component type magnetic toner which consists of an electroconductive magnetic toner. In this method, an electroconductive magnetic toner is attracted to and held on the surface of an electroconductive non-magnetic carrier sleeve having an inner magnet. By the relative movement of the carrier sleeve and the inner magnet, the toner held on the electroconductive non-magnetic carrier sleeve is brought into contact with latent electrostatic images formed on a latent electrostatic image bearing member which is backed with an electroconductive backing member. As a result, an electroconductive path is formed, which electrically connects the electroconductive backing member, the toner particles and the carrier sleeve, so that an electric charge having a polarity opposite to the polarity of the latent electrostatic images is induced in the magnetic toner and the latent electrostatic images are developed to visible toner images by the toner.
As disclosed, for example, in U.S. Pat. No. 3,639,245, an electroconductive magnetic toner employed in such a development method consists of toner particles whose surface portion is more electroconductive than the inner portion thereof. This electroconductive magnetic toner has the shortcoming that toner images developed by this magnetic toner are difficult to transfer to a recording medium, for instance, to an image transfer sheet, by an electrostatic image transfer method of applying an electric field to the toner images. In other words, the electrostatic image transfer performance of the toner is poor.
In order to improve the electrostatic image transfer performance, a magnetic toner having high electric resistivity has been proposed. Such a magnetic toner is in fact improved on the electrostatic image transfer performance, but has the shortcoming that image development performance is reduced due to the high resistivity of the toner.
Under such circumstance, there is disclosed in Japanese Laid-Open Patent Application No. 56-142540 a developer as being excellent in both development performance and image transfer, which consists of (i) magnetic toner particles having high electric resistivity, with magnetic powder being dispersed in each toner particle, and (ii) electroconductive magnetic particles having a mean volume diameter smaller than that of the toner particles. This developer, however, has the shortcoming that it is difficult to maintain a predetermined mixing ratio of the high resistivity magnetic toner particles and the electroconductive magnetic toner particles while in use.